Radio transceiver

ABSTRACT

A radio transceiver comprises a transmitter having an output circuit for transmitting signal information and a receiver having an input circuit for receiving signal information. An antenna is connected to the output circuit of the transmitter and to the input circuit of the receiver. A switching device is connected in series within the output circuit of the transmitter and in parallel across the input circuit of the receiver. When in one operative condition, the switching device enables the output circuit of the transmitter to transmit signal information to the antenna by short circuiting the input circuit of the receiver to disable the receiver. When in another operative condition, the switching device enables the input circuit of the receiver to receive signal information from the transmitter by open circuiting the output circuit of the transmitter to disable the transmitter.

United Mates Patent 3.lt2,445 ll/l963 Holdenetal............. 325/2lX[72] Inventor Donald E.Tomaszewskl G .m T d n a n e m n .l H m u C c e nes w .n R rwM M .IWRVM m m u hm myd mmmms m m .ISIT. TS! PAA n 0 u a r 0p r o C s 9 mt m umk 0 4 m unmm kl mmmm oo e xsF vcm M M; n ww muw AFPAABSTRACT: A radio transceiver comprises a transmitter having an outputcircuit for transmitting signal information and a receiver having aninput circuit for receiving signal information. An antenna is connectedto the output circuit of the transmitter and to the input circuit of thereceiver. A

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ing the input circuit of the receiver to disable the receiver. When inanother operative condition, the switching device S T N w MA 3. SE mT AT .NS D E W N U m U enables the input circuit of the receiver to receivesignal infor- 2,369,193 2/1945 Vrooman 325/21 mation from thetransmitter by open circuiting the output cir- 2,939,949 325/21X cuit ofthe transmitter to disable the transmitter.

6/1960 Curtis...................

RECEIVER ClRCUlTRY PATENTEUJUN 8197i GENERATOR INVISN'IYW. flozzald 5.Jomaszewsld ATTORIH Y RADIO TRANSCEIVER This invention relates to aradio transceiver, or more specifically, to a combination radiotransmitter and radio receiver.

A radio transceiver is a communications apparatus for transmitting andreceiving radio signals, usually in the citizens frequency band.Ordinarily, a radio transceiver includes a transmitter and a receiverwhich share a single antenna. Consequently, when it is desired totransmit, the antenna must be connected to the transmitter anddisconnected from the receiver. Conversely, when it is desired toreceive, the antenna must be connected to the receiver and disconnectedfrom the transmitter. The present invention proposes a radio transceiverwherein the antenna may be conveniently and efficiently switched betweenthe transmitter and the receiver.

According to one aspect of the invention, the antenna is permanentlyconnected to both the transmitter and the receiver. Further, a switchingdevice having a conductive condition and a nonconductive condition isconnected to the transmitter and the receiver. The switching device isoperative during one condition of conductivity to disable thetransmitter and enable the receiver thereby to effectively connect theantenna to the receiver only, and is operative during the othercondition of conductivity to disable the receiver and enable thetransmitter thereby to effectively connect the antenna to thetransmitter only.

In another aspect of the invention, the transmitter includes anoscillator having a tuned circuit, and the receiver includes an inputtransformer having a primary winding connected with the tuned circuit ofthe oscillator, The antenna is directly connected to the oscillator andis indirectly connected to the primary winding of the input transformerthrough the tuned circuit of the oscillator. The switching device isconnected in the tuned circuit of the oscillator and across the primarywinding of the input transformer. When the switching device is opened,the tuned circuit of the oscillator is open circuited so as to disablethe transmitter and enable the receiver thereby to effectively connectthe antenna through the tuned circuit to the primary winding of theinput transformer. When the switching device is closed, the primarywinding of the input transformer is short circuited so as to disable thereceiver and enable the transmitter thereby to effectively connect theantenna directly to the oscillator.

in a further aspect of the invention, the oscillator includes anamplifier device connected to a bias circuit and to the tuned circuit.The tuned circuit includes a tuning capacitor. The antenna is connecteddirectly to the amplifier device and indirectly to the primary windingof the input transformer through the tuning capacitor in the tunedcircuit. The switching device includes a switching transistor which isconnected in series within the bias circuit and the tuned circuit, andin parallel across the primary winding of the input transformer. When itis desired to receive signal information, the switching transistor isrendered relatively nonconductive to open circuit the tuned circuit andthe bias circuit of the oscillator thereby to effectively connect theantenna through the tuning capacitor to the primary winding of the inputtransformer. When it is desired to transmit signal information, theswitching transistor is rendered relatively conductive to short circuitthe primary winding of the input transformer thereby to effectivelyconnect the antenna directly to the amplifier device of the oscillator.

The invention may be best understood by reference to the followingdetailed description of a preferred embodiment when considered inconjunction with the accompanying drawing which is a schematic diagramof a radio transceiver incorporating the principles of the invention.

Referring to the drawing, the illustrated radio transceiver includes atransmitter for generating modulated radio signals and a receiver 12 fordemodulating modulated radio signals. An antenna 14 is directlyconnected to the transmitter 10 and is indirectly connected to thereceiver 12 through a portion of the transmitter 10. The antenna 14accepts modulated radio signals from the transmitter 10 and radiatesthem into the surrounding medium, and receives modulated radio signalsfrom the surrounding medium and couples them to the receiver 12. Aswitching device 16 having an opened condition and a closed condition isconnected to the transmitter 10 and across the receiver 12. In theopened condition, the switching device 16 is operative to open circuitthe transmitter 10 thereby to effectively connect the antenna 14 to thereceiver 12 for receiving modulated radio signals. In the closedcondition, the switching device 16 is operative to short circuit thereceiver 12 thereby to effectively connect the antenna to thetransmitter 10 for transmitting modulated radio signals.

The transmitter 10 includes an oscillator 18 for producing a radiofrequency carrier signal and a generator 20 for producing anaudiofrequency intelligence signal. The generator 20 is connected to theoscillator 18 so as to amplitude modulate the radio frequency carriersignal with the audiofrequency intelligence signal thereby to develop anamplitude modulated radio signal. The generator 20 may be any suitableapparatus capable of producing an audiofrequency intelligence signal tobe transmitted, as for example an audio pickup and amplifier device.

The oscillator 18 is a conventional Collpitts crystal oscillator. itincludes an amplifying component provided by a conventional junctiontransistor 22 having base, emitter and collector electrodes. Theoscillator 18 also comprises a bias circuit including biasing resistors24 and 26 connected in series between the positive side of a DC voltagesource 28 and ground. The DC voltage source 28 may be convenientlyprovided by a battery. The base electrode of the transistor 22 isconnected to the junction between the resistors 24 and 26. The biascircuit also includes a biasing resistor 30 and a choke coil 32connected in series between the emitter electrode of the transistor 22and the switching device 16.

The oscillator 18 further comprises a tuned circuit including a tuningcoil 34 connected in series with a bypass capacitor 36 between thecollector electrode of the transistor 22 and ground, a first tuningcapacitor 38 connected between the collector and emitter electrodes ofthe transistor 22, and a second tuning capacitor 40 connected betweenthe emitter electrode of the transistor 22 and the switching device 16.The tuned circuit also includes a piezoelectric crystal 42 connectedbetween the base electrode of the transistor 22 and ground. Thepiezoelectric crystal 42 may conveniently take the form ofa quartzcrystal. The tuned circuit of the oscillator 18 forms the output circuitof the transmitter 10.

The antenna 14 is permanently connected through a coupling capacitor 44to the emitter electrode of the transistor 22 in the oscillator 18. Theoutput of the generator 20 is connected to the oscillator 18 between thebiasing resistor 30 and the choke coil 32 in the bias circuit.

The receiver 12 includes an input transformer 46 having a primarywinding 48 and a secondary winding 50. The primary winding 48 isconnected in series with the second tuning capacitor 40 in the tunedcircuit and in series with the biasing resistor 30 in the bias circuitthrough a variable trimmer capacitor 52. The secondary winding 50 isconnected to receiver circuitry 54 which includes the remaining portionsof the receiver 12. Preferably, the receiver 12 is a conventional tunedfrequency receiver. The primary winding 48 of the input transfon'ner 46and the trimmer capacitor 52 form the input circuit of the receiver 12.

The switching device 16 includes a conventional junction transistor 56having base, emitter and collector electrodes. The emitter electrode ofthe transistor 56 is connected in series with the second tuningcapacitor 40 in the tuned circuit and the biasing resistor 40 in thebias circuit. The collector electrode of the transistor 56 is connecteddirectly to ground. Thus, the switching transistor 56 is effectivelyconnected in series within the tuned circuit and the bias circuit of theoscillator l8 and connected in parallel across the trimmer capacitor 52and the primary winding 48 of the input transformer 46. Put another way,the switching transistor 56 is connected in series with the outputcircuit of the transmitter and in parallel with the input circuit of thereceiver 12. A manually operable control switch 58 is connected betweenthe base and collector electrodes of the transistor 56 for controllingthe relative conductivity of the transistor 56.

in operation, when the control switch 58 is closed, the switchingtransistor 56 is turned on or rendered fully conductive. in the fullyconductive state, the transistor 56 acts as a short circuit.Consequently, the transistor 56 effectively connects the second tuningcapacitor 40 in the tuned circuit and the biasing resistor 30 in thebias circuit to ground thereby to enable the oscillator 13 or render itoperative, in addition, the transistor 56 places a short circuit acrossthe trimmer capacitor 52 and the primary winding 48 of the inputtransformer 46 so as to disable the input transformer 46 or render itinoperative. Thus, the antenna 14 is effectively coupled to theoscillator 18 of the transmitter 10 and is efi'ectively decoupled fromthe input transformer 46 of the receiver 12.

In the oscillator 18 the bias circuit fonned by the biasing resisters24, 26 and 30 establishes a DC bias voltage across the base and emitterelectrodes of the transistor 22 thereby to establish the operating pointof the transistor 22 and render it conductive. Preferably, thetransistor 22 is biased to provide Class C amplifier operation. Thetuned circuit formed by the tuning coil 34, the first and second tuningcapacitors 38 and 40, and the piezoelectric crystal 42 places thetransistor 22 in oscillation so as to produce a radio frequency carriersignal at the emitter electrode. The piezoelectric crystal 42 definesthe resonant frequency of the tuned circuit thereby to determine thefrequency of the radio frequency signal produced by the oscillator 18.

The generator 20 applies the audiofrequency intelligence signal acrossthe biasing resistor 30 so as to correspondingly vary the bias voltageacross the base and emitter electrodes of the transistor 22 thereby toamplitude modulate the radio frequency carrier signal with theaudiofrequency intelligence signal. The resultant amplitude modulatedradio signal appears at the emitter electrode of the transistor 22. Thismodulated radio signal is coupled through the capacitor 44 to theantenna 16 from which it is radiated into the surrounding medium.

The bypass capacitor 36 in the tuned circuit shunts radio frequencysignals across the biasing resistors 24 and 26 so as to prevent theradio frequency signals from influencing the bias voltage on thetransistor 22. Conversely, the choke coil 32 in the biasing circuitblocks radio frequency signals from feeding into the generator 20 so asto prevent the radio frequency signals from distorting theaudiofrequency signals produced by the generator 20. However, the chokecoil 32 passes audiofrequency signals so as to assure proper modulationof the radio frequency signal produced by the oscillator When thecontrol switch 58 is opened, the switching transistor 56 is turned offor rendered fully nonconductive. In the fully nonconductive state, thetransistor 56 acts as an open circuit. Consequently, the transistor 56places an open circuit across the trimmer capacitor 52 and the primarywinding 48 of the input transformer 46 thereby to enable the inputtransformer 46 or render it operative. In addition, the transistor 56effectively disconnects the second tuning capacitor 40 in the tunedcircuit and the biasing resistor 30 in the biasing circuit from groundso as to disable the oscillator 18 or render it inoperative. Thus, theantenna 14 is effectively coupled through the second tuning capacitor 40to the input transformer 46 of the receiver 12 and is effectivelydecoupled from the oscillator 18 of the transmitter 10. The trimmercapacitor 52 is adjusted so as to tune the primary winding 48 of theinput transformer 46 to the frequency of the modulated radio signal tohe received by the antenna 14. These modulated radio signals arereceived by the antenna 14 and coupled to the primary winding 48 of theinput transformer 46 through the coupling capacitor 44, the secondtuning capacitor 40 and the trimmer capacitor 52.

in the receiver 12, the modulated radio signals are transferred acrossthe input transformer 46 from the primary winding 48 to the secondarywinding 50 which applies them to the tuned frequency receiver circuitry54. Typically, the modulated radio signals are amplified and demodulatedin the receiver circuitry 54 so as to recover the audiofrequencyintelligence signal which is then amplified and applied to an audiooutput device such as a speaker.

In a radio transceiver constructed in accordance with the preferredembodiment of the invention, the following circuit components and valueswere found to yield satisfactory results:

Transistor 22 PT886 TRW Transistor 56 DS36 Delco Radio Resistor 24 2200ohms Resistor 26 1000 ohms 30 I50 ohms Capacitor 36 0.002 microfaradsCapacitor 38 47 picofarads Capacitor 40 l picofarads Capacitor 44 910picofarads Capacitor 52 30 picofarads lnductor 32 51 microhenries It isto be understood that the oscillator 18 need not be a Collpitt's crystaloscillator, but may be any suitable oscillator having a tuning capacitor40 through which the antenna 14 may be indirectly connected to theprimary winding 48 of the input transformer 46. Similarly, theamplifying component of the oscillator 18 need not be a junctiontransistor 22 but may be a field effect transistor, a vacuum tube, orany other suitable amplifier device. Likewise, the switching device 16need not include a junction transistor 56 but may utilize any suitableswitching element such as a field effect transistor, a vacuum tube, asilicon controlled rectifier, or a silicon controlled switch. Further,although the amplifying transistor 22 is illustrated as being of the NPNconductivity type and the switching transistor 56 is illustrated asbeing of the PNP conductivity type, it is to be noted that theamplifying transistor 22 and the switching transistor 56 may each be ofthe opposite conductivity type from that shown.

It will now be readily apparent that one particularly advantageousapplication of the invention is in a citizens band radio transceiversuch as the familiar walkie-talkie. Another advantageous application ofthe invention is in an automotive vehicle to supplement or replace theconventional vehicle horn. In the latter application, the generator 20may be provided by a tone signal source operated by the vehicle hornbutton which may serve as the control switch 56. The antenna 14 may beprovided by the conventional vehicle radio antenna and the audio outputdevice of the receiver circuitry 54 may be provided by the conventionalvehicle radio speaker. Thus, when the horn button is released,audiofrequency tone signals may be received from other automotivevehicles in the vicinity so as to energize the vehicle radio speaker toproduce a warning signal. Conversely, when the vehicle horn button isdepressed, an audio frequency tone signal may be transmitted from theantenna 14 to other automotive vehicles in the viciniy What I claim is:

1. A radio transceiver apparatus, comprising: a transmitter fortransmitting modulated radio signals; a receiver for receiving modulatedradio signals; an antenna connected to the transmitter for radiatingmodulated radio signals coupled from the transmitter and connected tothe receiver through a reactive portion of the transmitter for couplingreceived modulated radio signals to the receiver; and switching meansconnected to the transmitter and to the receiver and selectiveiyoperable for open circuiting the transmitter thereby to disable thetransmitter and effectively connect the antenna through the reactiveportion of the transmitter to the receiver, and for short circuiting thereceiver thereby to disable the receiver and effectively connect theantenna to the transmitter.

2. A radio transceiver apparatus, comprising: a transmitter having anoutput circuit for transmitting signal information; a receiver having aninput circuit for receiving signal information; an antenna connected tothe output circuit of the transmitter for transmitting signalinformation transferred to the antenna from the output circuit, andconnected to the input circuit of the receiver through a capacitiveportion of the output circuit of the transmitter for transferring signalinformation received by the antenna to the input circuit; switchingmeans having-a conductive condition and a nonconductive conditionconnected in series with the output circuit of the transmitter and inparallel across the input circuit of the receiver, so that in theconductive condition the switching means enables the output circuit ofthe transmitter to transmit signal information to the antenna by shortcircuiting the input circuit of the receiver to disable the receiver,and so that in the nonconductive condition the switching means enablesthe input circuit of the receiver to receive signal information from theantenna through the capacitive portion of the output circuit of thetransmitter by open circuiting the output circuit of the transmitter todisable the transmitter.

3. A radio transceiver apparatus, comprising: a transmitter including anoscillator for producing a radio frequency carrier signal which ismodulated by an audiofrequency intelligence signal applied to theoscillator, the oscillator including a tuned circuit for determining thefrequency of the radio frequency carrier signal; a receiver including aninput transformer having a primary winding connected in series with thetuned circuit of the oscillator; an antenna connected to the oscillatorfor radiating modulated radio signals developed by the transmitter andcoupled to the antenna, and connected to the primary winding of theinput transformer through a portion of the tuned circuit of theoscillator for coupling modulated radio signals received by the antennato the primary winding of the input transformer; and switching meansconnected within the tuned circuit of the oscillator for disabling thetransmitter by open circuiting the tuned circuit when the switchingmeans is opened thereby to effectively connect the antenna through theportion of the tuned circuit to the primary winding of the inputtransformer, and the switching means also connected across the primarywinding of the input transformer for disabling the receiver by shortcircuiting the primary winding of the input transformer when theswitching means is closed thereby to effectively connect the antenna tothe oscillator.

4. In combination: a transmitter comprising an amplifier device, a biascircuit connected to the amplifier device for establishing the operatingpoint of the amplifier device, a tuned circuit including a tuningcapacitor connected to the amplifier device for placing the amplifierdevice in oscillation so as to produce a radio frequency carrier signal,and generator means connected to the amplifier device for applying anaudiofrequency intelligence signal to the amplifier device thereby toamplitude modulate the carrier signal with the intelligence signal so asto obtain an amplitude modulated radio signal; a receiver including aninput transformer having a primary winding connected in the bias circuitand to the tuning capacitor in the tuned circuit; an antenna connectedto the amplifier device for radiating modulated radio signals coupledfrom the amplifier device, and connected to the primary winding of theinput transformer through the tuning capacitor in the tuned circuit forcoupling received modulated signals to the primary winding of the inputtransformer; and switching means connected in the bias circuit and inthe tuned circuit for rendering the transmitter inoperative by opencircuiting the bias circuit and the tuned circuit when the switchingmeans is opened thereby to effectively connect the antenna through thetuning capacitor in the tuned circuit to the primary winding of theinput transformer, and the switching means also connected across theprimary winding of the input transformer for rendering the receiverinoperative by short circuiting the primary winding of the inputtransformer when the switching means is closed thereby to effectivelyconnect the antenna to the amplifier device. 5. In combination: a radiotransmitter comprlsing an amplifying transistor, a bias circuitconnected to the amplifying transistor for establishing the operatingpoint of the amplifying transistor, a tuned circuit including a tuningcapacitor connected to the amplifying transistor for placing theamplifying transistor in operation so as to produce a radio frequencycarrier signal, and generator means connected to the amplifyingtransistor for applying an audiofrequency intelligence signal to theamplifying transistor thereby to amplitude modulate the carrier signalwith the intelligence signal so as to obtain an amplitude modulatedradio signal; a receiver including an input transformer having a primarywinding connected in the bias circuit and to the tuning capacitor in thetuned circuit; an antenna connected to the amplifying transistor forradiating modulated radio signals coupled from the amplifyingtransistor, and connected to the primary winding of the inputtransformer through the tuning capacitor in the tuned circuit forcoupling received modulated radio signals to the primary winding of theinput transformer; and switching means including a switching transistorconnected in series within the bias circuit and the tuned circuit andconnected in parallel across the primary winding of the inputtransformer, the switching means further including a manually operableswitch connected to the switching transistor for rendering the switchingtransistor nonconductive to open the bias circuit and the tuned circuitthereby to effectively connect the antenna through the tuning capacitorin the tuned circuit to the primary winding of the input transformer,and for rendering the switching transistor conductive to short theprimary winding of the input transformer thereby to effectively connectthe antenna to the amplifying transistor.

1. A radio transceiver apparatus, comprising: a transmitter fortransmitting modulated radio signals; a receiver for receiving modulatedradio signals; an antenna connected to the transmitter for radiatingmodulated radio signals coupled from the transmitter and connected tothe receiver through a reactive portion of the transmitter for couplingreceived modulated radio signals to the receiver; and switching meansconnected to the transmitter and to the receiver and selectivelyoperable for open circuiting the transmitter thereby to disable thetransmitter and effectively connect the antenna through the reactiveportion of the transmitter to the receiver, and for short circuiting thereceiver thereby to disable the receiver and effectively connect theantenna to the transmitter.
 2. A radio transceiver apparatus,comprising: a transmitter having an output circuit for transmittingsignal information; a receiver having an input circuit for receivingsignal information; an antenna connected to the output circuit of thetransmitter for transmitting signal information transferred to theantenna from the output circuit, and connected to the input circuit ofthe receiver through a capacitive portion of the output circuit of thetransmitter for transferring signal information received by the antennato the input circuit; switching means having a conductive condition anda nonconductive condition connected in series with the output circuit ofthe transmitter and in parallel across the input circuit of thereceiver, so that in the conductive condition the switching meansenables the output circuit of the transmitter to transmit signalinformation to the antenna by short circuiting the input circuit of thereceiver to disable the receiver, and so that in the nonconductivecondition the switching means enables the input circuit of the receiverto receive signal information from the antenna through the capacitiveportion of the output circuit of the transmitter by open circuiting theoutput circuit of the transmitter to disable the transmitter.
 3. A radiotransceiver apparatus, comprising: a transmitter including an oscillatorfor producing a radio frequency carrier signal which is modulated by anaudiofrequency intelligence signal applied to the oscillator, theoscillator including a tuned circuit for determining the frequency ofthe radio frequency carrier signal; a receiver including an inputtransformer having a primary winding connected in series with the tunedcircuit of the oscillator; an antenna connected to the oscillator forradiating modulated radio signals developed by the transmitter andcoupled to the antenna, and connected to the primary winding of theinput transformer through a portion of the tuned circuit of theoscillator for coupling modulated radio signals received by the antennato the primary winding of the input transformer; and switching meansconnected within the tuned circuit of the oscillator for disabling thetransmitter by open circuiting the tuned circuit when the switchingmeans is opened thereby to effectively connect the antenna through theportion of the tuned circuit to the primary winding of the inputtransformer, and the switching means also connected across the primarywinding of the input transformer for disabling the receiver by shortcircuiting the primary winding of the input transformer when theswitching means is closed thereby to effectively connect the antenna tothe oscillator.
 4. In combination: a transmitter comprising an amplifierdevice, a bias circuit connected to the amplifier device forestablishing the operating point of the amplifier device, a tunedcircuit including a tuning capacitor connected to the amplifier devicefor placing the amplifier device in oscillation so as to produce a radiofrequency carrier signal, and generator means connected to the amplifierdevice for applying an audiofrequency intelligence signal to theamplifier device thereby to amplitude modulate the carrier signal withthe intelligence signal so as to obtain an amplitude modulated radiosignal; a receiver including an input transformer having a primarywinding connected in the bias circuit and to the tuning capacitor in thetuned circuit; an antenna connected to the amplifier device forradiating modulated radio signals coupled from the amplifier device, andconnected to the primary winding of the input transformer through thetuning capacitor in the tuned circuit for coupling received modulatedsignals to the primary winding of the input transformer; and switchingmeans connected in the bias circuit and in the tuned circuit forrendering the transmitter inoperative by open circuiting the biascircuit and the tuned circuit when the switching means is opened therebyto effectively connect the antenna through the tuning capacitor in thetuned circuit to the primary winding of the input transformer, and theswitching means also connected across the primary winding of the inputtransformer for rendering the receiver inoperative by short circuitingthe primary winding of the input transformer when the switching means isclosed thereby to effectively connect the antenna to the amplifierdevice.
 5. In combination: a radio transmitter comprising an amplifyingtransistor, a bias circuit connected to the amplifying transistor forestablishing the operating point of the amplifying transistor, a tunedcircuit including a tuning capacitor connected to the amplifyingtransistor for placing the amplifying transistor in operation so as toproduce a radio frequency carrier signal, and generator means connectedto the amplifying transistor for applying an audiofrequency intelligencesignal to the amplifying transistor thereby to amplitude modulate thecarrier signal with the intelligence signal so as to obtain an amplitudemodulated radio signal; a receiver including an input transformer havinga primary winding connected in the bias circuit and to the tuningcapacitor in the tuned circuit; an antenna connected to the amplifyingtransistor for radiating modulated radio signals coupled from theamplifying transistor, and connected to the primary winding of the inputtransformer through the tuning capacitor in the tuned circuit forcoupling received modulated radio signals to the primary winding of theinput transformer; and switching means including a switching transistorconnected in series within the bias circuit and the tuned circuit andconnected in parallel across the primary winding of the inputtransformer, the switching means further including a manually operablesWitch connected to the switching transistor for rendering the switchingtransistor nonconductive to open the bias circuit and the tuned circuitthereby to effectively connect the antenna through the tuning capacitorin the tuned circuit to the primary winding of the input transformer,and for rendering the switching transistor conductive to short theprimary winding of the input transformer thereby to effectively connectthe antenna to the amplifying transistor.